Saving a Custom Post Type with a Meta Box results in a “Updating Failed” error

I changed your code and its now working fine in my pc.
here is the code

<?php

    function declare_custom_post_type() {
        $theme_name="thatannalam";
        $custom_post_type="favourites";

        $labels = array(
            'name'               => __('Favourites', $theme_name),
            'singular_name'      => __('Favourite', $theme_name),
            'add_new'            => __( 'Add New Favourite' ),
            'add_new_item'       => __( 'Add New Favourite' ),
            'edit_item'          => __( 'Edit Favourite' ),
            'new_item'           => __( 'Add New Favourite' ),
            'view_item'          => __( 'View Favourite' ),
            'search_items'       => __( 'Search EvFavouriteent' ),
            'not_found'          => __( 'No favourites found' ),
            'not_found_in_trash' => __( 'No favourites found in trash' )
        );

        $args = array(
            'description' => __( 'Movie news and reviews', $theme_name),
            'exclude_from_search' => false,
            'labels' => $labels,
            'has_archive' => true,
            'hierarchical' => false,
            'public' => true,
            'register_meta_box_cb' => 'register_meta_box',
            'rewrite' => array('slug', $custom_post_type),
            'show_in_admin_bar' => true,
            'show_in_menu' => true,
            'show_in_nav_menus' => true,
            'show_in_rest' => true,
            'show_ui' => true,
            'supports' => array('title', 'editor', 'custom-fields')
        );

        register_post_type($custom_post_type, $args);
    }
    add_action('init', 'declare_custom_post_type');

    function register_meta_box() {
        add_meta_box(
            'favourite_details_box',
            'Favourite Details',
            'render_meta_html',
            'favourites',
            'side'
        );
    }

    function render_meta_html() {
        global $post;
        $nonce_key = 'favourite_fields_nonce';
        $link_meta_key = 'product_link_key';

        wp_nonce_field($nonce_key, $nonce_key);

        $link = get_post_meta( $post->ID, $link_meta_key, true );
        echo '<label for="product_link_field" class="widefat">Product Link</label><input type="text" id="product_link_field" name="product_link_field" class="widefat" value="'.$link.'">';
    }

    function save_favourite_meta($post_id, $post) {
        $nonce_key = 'favourite_fields_nonce';
        $link_meta_key = 'product_link_key';
        $link_field_name="product_link_field";
        $custom_post_type="favourites";

        // verify nonce
        if (!isset($_POST[$nonce_key]) || !wp_verify_nonce($_POST[$nonce_key], $nonce_key))
            return 'nonce not verified';

        // check autosave
        if (wp_is_post_autosave($post_id ))
            return 'autosave';

        //check post revision
        if (wp_is_post_revision($post_id))
            return 'revision';

        // check permissions
        if ( ! current_user_can( 'edit_post', $post_id ) ) {
            return 'cannot edit post';
        }

        $product_link = $_POST[$link_field_name];            
            update_post_meta($post_id, $link_meta_key, $product_link);        
    }

    add_action( 'save_post', 'save_favourite_meta', 10,2);
?>

I changed 2 things:

1. we must have to pass a number of arguments to add_action() function.
2. only used update_post_meta() function.

Be careful if you want to use update_post_meta() and add_post_meta() both together. because in your code it saves value multiple times in the database.